1. Field of the Invention
The present invention relates to a solenoid driving device and a solenoid valve.
2. Description of the Related Art
Conventionally, for example, various types of solenoid valves may be disposed in a hydraulic circuit of an automatic transmission. Each of the solenoid valves may have a solenoid unit made up of a solenoid driving device and a valve unit. The valve unit may be operated by supplying electric current to a coil of the solenoid unit, thereby opening and closing the pneumatic paths, adjusting the flow of oil, and regulating the oil pressure.
Next, a solenoid valve, using as an example a linear solenoid valve, will now be described. FIG. 2 is a cross-sectional view of a conventional linear solenoid valve. In the drawing, reference numeral 11 denotes a solenoid unit, and reference numeral 12 denotes a regulator valve unit made up of a valve unit operated by driving the solenoid unit 11. The solenoid unit 11 comprises a coil assembly 13, a plunger 14 which is disposed so as to advance toward and retreat from the coil assembly 13 (i.e., movement in the horizontal direction in the drawing), and a yoke 20 disposed so as to enclose the coil assembly 13. Also, the coil assembly 13 has a coil 17 formed by winding a coil wire 16 on a bobbin 15, end portions 18 and 19 disposed on both ends of the coil 17, and a terminal 21 for supplying electric current to the coil 17, wherein the bobbin 15 and end portions 18 and 19 are integrally assembled by welding, brazing, sintered bonding, adhesion, or the like.
A thick flange portion 28 having a tapered shape on the inner side thereof is formed on the regulator valve 12 side of the bobbin 15. A tapered edge portion 31 protrudes backwards (toward the right in the drawing) from near the inner circumference edge of the end portion 19 so as to correspond to the inner circumference face of the flange portion 28.
The coil assembly 13 is formed cylindrically except for the terminal 21 portion, with a hollow portion 22, having a constant diameter in the axial direction, formed within the coil assembly (within the bobbin 15 and end portions 18 and 19) and the plunger 14 slideably fit into the hollow portion 22. The plunger 14 has a uniform diameter and is longer than the coil 17 in the axial direction.
The rear end (the right end in the drawing) of the spool 26 of the regulator valve 12 is brought into contact with the center of the front edge face (the left end in the drawing) S1 of the plunger 14, and a contact portion 27 with a spherical shape having a predetermined height is integrally formed on the edge face at the side away from the regulator valve 12, i.e., the rear edge face (the right end in the drawing) S2.
The bobbin 15 is formed of a non-magnetic member, and, upon electric current being supplied to the coil 17 at the solenoid unit 11, a magnetic flux is generated so that a magnetic path is formed from the yoke 20 through the end portion 18, plunger 14 and end unit 19 and back to the yoke 20, whereby a force S is formed between the edge portion 31 and the plunger 14 in the magnetic path.
The force generated by coil 17 then acts on the plunger 14 with a predetermined force, providing the plunger 14 with thrust. Consequently, the thrust is transmitted to the spool 26, the regulator valve 12 is operated, and the oil pressure is regulated.
When thrust is generated at the plunger 14 upon electric current being supplied to the coil wire 16, thereby advancing or retracting the plunger 14 within the hollow portion 22, friction is generated between the outer face of the plunger 14 and the inner face of the coil assembly 13 because the plunger 14 is supported by the coil assembly 13 when it is fit into the hollow portion 22. Accordingly, the outer face of the plunger 14 is processed so as to form an outer layer with a small friction coefficient μ. Also, the outer layer is formed with a non-magnetic material such that fine foreign matter, such as iron powder or the like, does not intrude between the outer face of the plunger 14 and the inner face of the coil assembly 13 (e.g., see Japanese Unexamined Patent Application Publication No. 2003-134781).
However, with conventional linear solenoid valves, there is the need to use fluororesin (PTFE: PolyTetra Fluoro Ethylene), molybdenum disulfide, amorphous carbon (DLC), or the like as a non-magnetic member having a small friction coefficient μ. Therefore, there is a limit as to how thick the film can be formed for the outer layer.
Accordingly, it is difficult to sufficiently sever the magnetism between the plunger 14 and end portions 18 and 19, therefore, the force against the plunger 14 in the radial direction (side force) cannot be sufficiently prevented.
In this case, the plunger 14 does not slide well through the coil assembly 13, therefore, the plunger 14 cannot advance and retreat smoothly, oil pressure cannot be accurately regulated with the regulator valve 12, and consequently, the performance of the linear solenoid valve deteriorates.